1. Field of the Invention
The present invention relates to an exhaust outlet structure for a personal watercraft. More specifically, the present invention relates to an exhaust outlet structure for discharging the exhaust gas of an engine mounted on the personal watercraft.
2. Description of the Related Art
In general, the personal watercraft is provided with an exhaust outlet for discharging an exhaust gas from an engine at a rear portion of a hull. Some personal watercrafts serve to discharge cooling water of an engine and an exhaust system together with the exhaust gas from an exhaust outlet to a rear outside by a pressure (back pressure) of the exhaust gas. However, the personal watercraft is a vehicle to plane along the water surface. Therefore, the exhaust outlet is positioned on the water surface during running and is sometimes positioned below the water surface at the time of stop. Accordingly, if the exhaust outlet is kept open, there is a possibility that the water might flow from the outside to the inside through the exhaust outlet to cause a counterflow when the engine is stopped so as to prevent the discharge of any exhaust gas.
As shown in the prior art configuration of FIG. 8, conventionally, exhaust pipes 51 and 52 constituting exhaust passages in a personal watercraft S and a water muffler 53 for noise elimination have formed a maze. First of all, the water muffler 53 is divided into right and left chambers 53a and 53b by a bulkhead 54. A communicating pipe 55 for causing the chambers 53a and 53b to communicate with each other is provided slightly above the center of the bulkhead 54. The exhaust pipe 51 connected from an engine E to the water muffler 53 has a rear end 51a opened in the vicinity of a central portion of the front chamber 53a of the water muffler 53. The exhaust pipe 52 connected from the water muffler 53 to an exhaust outlet 56 provided in a rear portion of a hull of the watercraft S has a front end 52a opened in the vicinity of a central portion of the rear chamber 53b of the water muffler 53. Furthermore, the exhaust pipe 52 extends upward from the front end 52a thereof through the water muffler 53, and is then curved downward and extends rearward. Thus, a countermeasure has been taken such that the water stays in the bottom of the water muffler 53 so as not to cause a counterflow to the engine side even if it flows to the inside through the exhaust outlet 56 when the engine is stopped.
Since the cooling water is mixed in the exhaust gas to perform cooling of the exhaust gas, the water muffler 53 promotes a reduction in exhaust sounds. For this reason, a duplex pipe is usually used as the exhaust pipe 51 reaching the water muffler 53, and a portion between an outer pipe and inner pipe through which the exhaust gas passes, is formed as a passage for the cooling water. A rear end of the exhaust pipe 51 is opened in the water muffler 53, and the cooling water is mixed with the exhaust gas. The bulkhead 54 has a small communicating hole 58 such that the cooling water and the exhaust gas can flow into the chambers 53b from the chambers 53a.
When the watercraft S planes, a cavity V is generated on a water surface portion in a rear part of the watercraft S as shown in FIG. 8. Consequently, reflected sounds of the exhaust gas are made by a water surface F. In some cases, a nonreturn valve (also referred to as a bellows valve) 57 having a simple structure is provided on the exhaust outlet 56 for guiding an exhaust flow to the water surface as shown by an arrow A in order to control the reflected sounds.
In the drawing, the reference numeral 59 denotes a steering handlebar, the reference numeral 60 denotes a seat on which a driver sits, and the reference numeral 61 denotes a fuel tank.
Japanese Unexamined Utility Model Applications Nos. Hei 2-134000 and Hei 5-34100 have disclosed the exhaust outlet of the personal watercraft.
The water muffler 53 forms a part of the exhaust passage, and is greatly concerned with the performance of the engine. Accordingly, it is desirable that the exhaust passage provided in the water muffler 53 should have a shape which is as simple as possible in respect of an enhancement in the output performance of the engine and simplification of a performance test in the engine.
Although the nonreturn valve 57 has some effects of controlling the reflected sounds, sealing against the water on the outside is not sufficiently performed.